Natural gas is transported long distances in a gaseous state to consumers through a gas pipe line over land or sea, or is transported in an LNG state by LNG carriers. LNG is obtained by cooling natural gas into a cryogenic state (about −163) where the volume of the natural gas is reduced to about 1/600 that at standard temperature and pressure, which makes it eminently suitable for long distance marine transportation.
Recently, demand for floating offshore structures such as LNG FPSO (floating, production, storage and offloading), LNG FSRU (floating storage and regasification unit) or the like have been increasing. The floating offshore structure is also provided with storage tanks which are provided to LNG carriers or LNG RVs (regasification vessels), and, if necessary, with an LNG liquefaction or regasification system.
The LNG FPSO is a floating offshore structure that can directly liquefy produced natural gas into LNG at sea to store the LNG in the storage tanks thereof and to deliver the LNG stored in the storage tanks to another LNG carrier, as needed. The LNG FSRU is a floating offshore structure that can store LNG, unloaded from an LNG carrier, in the storage tank at sea a long distance from the land and can gasify the LNG as needed, thereby supplying the regasified LNG to consumers on the land.
The floating offshore structures, such as LNG FPSO, LNG FSRU and the like, which can store liquid cargo such as LNG on the sea, are generally provided as barges, which are anchored by a single-point mooring system such as turret or Yoke mooring.
However, such a barge type floating offshore structure is sensitive to marine conditions and is likely to fluctuate according to the marine conditions, and particularly suffers a sloshing phenomenon within the storage tanks due to roll movement of the offshore structure and workability deterioration due to large relative movement with respect to an LNG carrier near the floating offshore structure during an offloading operation.
Excessive fluctuation of the floating offshore structure, sloshing in the storage tank, and resonance motion of the offshore structure can make it difficult to operate various systems provided to the offshore structure or can cause severe sloshing in the storage tank. Herein, “sloshing” refers to movement of a liquid substance, that is, LNG, stored in the storage tank when a floating structure such as vessels moves in various marine conditions, and imparts severe impact to a wall of the storage tank.
In particular, since partial loading of a liquid cargo in the storage tank causes a severer sloshing phenomenon, the LNG carrier regulates a loading amount of LNG to be stored in a cargo tank. However, since it is difficult for the floating offshore structure to regulate the loading amount of LNG in the storage tank thereof, the floating offshore structure inevitably suffers impact caused by sloshing upon partial loading of LNG.
Conventionally, in use of membrane type storage tanks which have a relatively low strength, it has been proposed to arrange the membrane type storage tanks in two arrays with a partition disposed therebetween. Alternatively, it has been proposed to use independent type storage tanks. However, the arrangement of the membrane type storage tanks in two arrays leads to a decrease of storage capacity and the use of the independent type storage tanks require an increase of manufacturing costs.
On the other hand, when LNG is supplied from the floating offshore structure to the LNG carrier (in the case of the LNG FPSO) or vice versa (in the case of the LNG FSRU), it is necessary for the LNG carrier to be placed near the offshore structure. However, since the conventional floating offshore structure has the barge structure that is sensitive to marine conditions and fluctuates as described above, the conventional floating offshore structure provides low workability during LNG offloading due to large relative movement with respect to the LNG carrier placed adjacent thereto.